Capacitive load driving unit and method and apparatus for inspecting the same

ABSTRACT

A capacitive load driving unit includes a plurality of capacitive loads, a drive circuit for charging and discharging the capacitive loads, a pair of power lines for supplying a drive voltage from an external power source to the drive circuit, and a current reserve circuit for reserving a current required by the drive circuit for charging the capacitive loads. The current reserve circuit includes a capacitor that is charged by the drive voltage from the power lines, and a coupler section for coupling the capacitor between the power lines except for when a defect inspection is conducted.

[0001] This is a Division of application Ser. No. 09/372,820 filed Aug.11, 1999.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a capacitive load driving unitintegrally incorporating a capacitive load such as a print head for anink-jet printer and a drive circuit for the capacitive load, and amethod and apparatus for inspecting the unit.

[0003] The print head of a typical ink-jet printer has a plurality ofink chambers arranged in a line and partitioned by a plurality ofelectrostrictive members. Each of the ink chambers jets or ejects dropsof ink from an ink-jet nozzle thereof upon a change in pressure causedwhen a corresponding electrostrictive member deforms according to avoltage applied to a pair of electrodes formed on both sides of theelectrostrictive member. The pair of electrodes and the electrostrictivemember constitute a capacitive load.

[0004]FIG. 21 shows an example of a drive circuit for driving such acapacitive load. This drive circuit includes a pair of transistors Tr1and Tr2 and a level converter L. A capacitive load H is connectedbetween a ground terminal GND and output terminal OUT of the 25 drivecircuit. The transistor Tr1 is connected between a source voltageterminal VD and the output terminal OUT. The transistor Tr2 is connectedbetween the output terminal OUT and the ground terminal GND. The levelconverter L generates voltage signals for alternately turning on thesetransistors Tr1 and Tr2. The voltage across the capacitive load H is setto a level equal to that of the source voltage terminal VD by a chargecurrent flowing from the source voltage terminal VD to the capacitiveload H through the transistor Tr1 when the transistor Tr1 is turned on.In addition, the voltage across the capacitive load H is set to a levelequal to that of the ground terminal GND by a discharge current flowingfrom the capacitive load H to the ground terminal GND through thetransistor Tr2 when the transistor Tr2 is turned on.

[0005] When the above print head and drive circuit are to bemanufactured as a single head unit, the connection state between thehead and drive circuit is inspected or checked to determine whether thehead unit is nondefective or defective. However, the print head and thedrive circuit are interconnected by a large number of wiring lines suchas about 100 to 3000 wiring lines which are very thin and arranged at asmall pitch of about 50 to 200 μm. For these reasons, it is nearlyimpossible that a probe for picking up a current or voltage issequentially set to be in contact with the wiring lines when the printhead is driven.

[0006] Under the circumstances, Jpn. Pat. Appln. KOKAI Publication No.10-86358 discloses a head unit 4 which is, as shown in FIG. 22,comprised of a unit interface 1, drive circuit 2, and print head 3, anda solder point 7. In the head unit 4, the solder point 7 is formed as amelt coupler between a drive circuit ground line 5 and a head groundline 6. A waveform of a current flowing from the print head 3 to thehead ground line 6 is detected with a current probe 8 while the solderpoint 7 is set in a disconnected state, and this current waveform isconverted into a voltage waveform by a current-voltage converter. Thevoltage waveform is integrated by an integrating device and supplied toa waveform recorder. The waveform recording unit records this waveform.The recorded waveform is compared with a normal waveform to determinewhether the print head unit 4 is nondefective or defective. After theinspection, the solder point 7 is melted for short-circuiting so as toprevent currents for charging and discharging the print head 3 fromflowing outside the head unit 4.

[0007] A power source voltage is applied from an external power sourceto a head unit via a relatively long cable. In a case where the printhead is driven at a high speed, the inductance component of the cablegreatly influences the current-supply ability of the external powersource, so that current supply cannot follow the charging cycle of thecapacitive load. To cope with the problem, the head unit 4 may have abypass capacitor which is connected between the power lines 5 and 6 andlocated near-the drive circuit 2 to be subjected to almost no influenceof the inductance component of the cable. The capacitor is charged bythe drive voltage applied from the external power source to reserve acurrent required by the drive circuit 2 for charging the capacitiveload.

[0008] When the capacitor is incorporated in the head unit 4 of thereference and the waveform of a current flowing from the print head 3 tothe head ground line 6 is detected as described above, the currentwaveform cannot be correctly detected owing to the influence of thecapacitor. As a result, whether the head unit is nondefective ordefective cannot be accurately determined.

BRIEF SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a capacitiveload driving unit, an inspection method and an inspection apparatus forthe unit, which permit a defect inspection to be performed without beinginfluenced by a capacitor for reserving a current for charging acapacitive load.

[0010] According to the present invention, there is provided acapacitive load driving unit which comprises a plurality of capacitiveloads, a drive circuit for charging and discharging the capacitiveloads, a pair of power lines for supplying a drive voltage from anexternal power source to the drive circuit, and a current reservecircuit for reserving a current required by the drive circuit forcharging the capacitive loads, wherein the current reserve circuitincludes a capacitor that is charged by the drive voltage from the powerlines, and a coupler section for coupling the capacitor between thepower lines except for when a defect inspection is conducted.

[0011] Further, according to the present invention, there is provided aninspection method for a capacitive load driving unit which comprises aplurality of capacitive loads, a drive circuit for charging anddischarging the capacitive loads, a pair of power lines for supplying adrive voltage from an external power source to the drive circuit, and acurrent reserve circuit for reserving a current required by the drivecircuit for charging the capacitive loads, wherein the current reservecircuit includes a capacitor that is charged by the drive voltage fromthe power lines, and a coupler section for coupling the capacitorbetween the power lines except for when a defect inspection isconducted, the method comprising the steps of controlling the drivecircuit to sequentially drive the plurality of capacitive loads in astate where the capacitor is disconnected from the power lines; anddetecting an electric change that occurs on the power line upon drivingeach of the capacitive loads to determine whether or not a defect ispresent.

[0012] Moreover, according to the present invention, there is providedan inspection apparatus for a capacitive load driving unit whichcomprises a plurality of capacitive loads, a drive circuit for chargingand discharging the capacitive loads, a pair of power lines forsupplying a drive voltage from an external power source to the drivecircuit, and a current reserve circuit for reserving a current requiredby the drive circuit for charging the capacitive loads, wherein thecurrent reserve circuit includes a capacitor that is charged by thedrive voltage from the power lines, and a coupler section for couplingthe capacitor between the power lines except for when a defectinspection is conducted, the apparatus comprising a control circuit forcontrolling the drive circuit to sequentially drive the plurality ofcapacitive loads in a state where the capacitor is disconnected from thepower lines, and a detecting circuit for detecting an electric changethat occurs on the power line upon driving each of the capacitive loadsto determine whether or not a defect is present.

[0013] In the capacitive load driving unit, the inspection method, andthe inspection apparatus, the coupler section causes the capacitor forreserving a charging current for the capacitive loads to be disconnectedfrom the power lines in a defect inspection. Therefore, the defectinspection can be performed without being influenced by the capacitor.

[0014] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0015] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiments of the invention, and together with the general descriptiongiven above and the detailed description of the preferred embodimentsgiven below, serve to explain the principles of the invention.

[0016]FIG. 1 is a block diagram showing a head unit for an ink-jetprinter according to an embodiment of the present invention;

[0017]FIG. 2 is a block diagram showing the arrangement of a drivecircuit shown in FIG. 1;

[0018]FIG. 3 is a block diagram showing the arrangement of a controlcircuit for controlling the drive circuit shown in FIG. 1;

[0019]FIGS. 4 and 5 are timing charts for explaining the operation ofthe control circuit shown in FIG. 3;

[0020]FIGS. 6A to 6D are views showing examples of current waveformsdisplayed on the display screen of the oscilloscope shown in FIG. 1;

[0021]FIG. 7 is a view showing the detailed structure of the head unitshown in FIG. 1;

[0022] FIGS. 8 to 11 are views showing modifications of the head unitshown in FIG. 7;

[0023]FIGS. 12A to 12C are views showing examples of cables which areconnected to the head unit shown in FIG. 11 during and after a defectinspection;

[0024]FIG. 13 is a view showing a modification of the head unit shown inFIG. 7;

[0025]FIG. 14 is a view showing a state where a capacitor is mounted onthe head unit shown in FIG. 13;

[0026]FIG. 15 is an equivalent circuit diagram of the ink-jet print headshown in FIG. 1;

[0027]FIGS. 16 and 17 are equivalent circuit diagrams of modificationsof the ink-jet print head shown in FIG. 1;

[0028]FIGS. 18 and 19 are block diagrams showing modifications of aninspection apparatus shown in FIG. 1;

[0029]FIGS. 20A to 20G are views showing the relationship between thecurrent waveforms displayed on the oscilloscope and a predeterminedallowable range;

[0030]FIG. 21 is an equivalent circuit diagram of a drive circuit forcharging and discharging a capacitive load of an ink-jet print head; and

[0031]FIG. 22 is a block diagram for explaining a conventionalwaveform-recording scheme.

DETAILED DESCRIPTION OF THE INVENTION

[0032] A head unit for an ink-jet printer according to an embodiment ofthe present invention will be described with reference to the views ofthe accompanying drawing.

[0033]FIG. 1 shows the head unit 11 for the ink-jet printer. This headunit 11 includes an ink-jet print head 12 and a drive circuit 13 whichare formed in one unit. The print head 12 has a plurality of inkchambers arranged in a line and partitioned by a plurality ofelectrostrictive members. Each of the ink chambers jets or ejects dropsof ink from an ink-jet nozzle thereof upon a change in pressure causedby deformation of a corresponding electrostrictive member. Each of theelectrostrictive members is formed between a pair of electrodes toconstitute a capacitive load that deforms according to a voltage appliedto the pair of electrodes. The drive circuit 13 is arranged to charge ordischarge the capacitive loads by a drive voltage supplied thereto viapower lines Vcc and Vss. The head unit 11 further includes a currentreserve circuit for reserving a current required by the drive circuit 13to charge the capacitive loads. The current reserve circuit has acapacitor 14 which is charged by the drive voltage supplied via thepower lines Vcc and Vss and a coupler section 15 for coupling thecapacitor 14 between the power lines Vcc and Vss except for a defectinspection. The capacitor 14 has a function of instantaneously supplyinga charge current for the capacitive loads to the drive circuit 13 andalso bypassing a power source noise. To attain this function, thecapacitor 14 has a capacitance which is 200 times the sum of thecapacitances of all the capacitive loads. In a case where the print head12 is comprised of 100 capacitive loads each having a capacitance of 500pF, the capacitor 14 is determined to have a capacitance of 10 μF.

[0034] The print head 12 and the drive circuit 13 are interconnected bya large number of wiring lines such as about 100 to 300 wiring linesformed of TABs, bonding wires, or the like. These wiring lines are verythin and arranged at a small pitch of about 50 to 200 μm.

[0035] The head unit 11 further includes a connector 16 which is usedfor detachably connecting the drive circuit 13 to the externalinspection apparatus. This inspection apparatus includes a regulatedpower source for supplying a power source voltage as the drive voltageto the drive circuit 13 via the power lines Vcc and Vss, a controlcircuit 18 for supplying an enable signal, a latch signal, data signal,and a shift clock signal to the drive circuit via signal lines, a 5-Vpower source 19 for supplying a power source voltage of 5V via powerlines Vdd and Vss to the drive circuit 13, and a power line capacitor 20connected between the power lines Vcc and Vss and having a capacitancesubstantially equal to that of the capacitor 14. The capacitor 14 islocated at a position closer to the drive circuit 13 than the connector16.

[0036]FIG. 2 shows the arrangement of the drive circuit 13. The drivecircuit 13 is comprised of a buffer 131, gate circuit 132, latch circuit133, and shift register 134. Data from the control circuit 18 are storedin the shift register 134 while sequentially shifted in synchronism witha shift clock. When a series of data for driving the ink-jet print head12 is stored in the shift register 134, a latch signal from the controlcircuit 18 is input to the latch circuit 133 to latch the data itemsfrom the shift register 134 in the latch circuit 133. When an enablesignal from the control circuit 18 is activated afterward, the gatecircuit 132 operates to supply the data items latched by the latchcircuit 133 to the buffer 131. As a consequence, the drive voltage isapplied to the capacitive loads for the ink chambers of the print head13 on the basis of the data items.

[0037] A current probe 211 of an oscilloscope 21 for recording a currentwaveform is arranged between the capacitor 20 and the connector 16 onthe power line Vcc which are connected to the voltage regulated powersource 17 and the connector 16. In addition, a voltage probe 212 of theoscilloscope 21 is connected between the enable signal line and thepower line Vss. With this arrangement, the oscilloscope 21 detects thewaveform of a current flowing through the power line Vcc in response toan enable signal serving as a trigger, and displays the current waveformon the display screen.

[0038] Since a large pulse current flows into the head unit 11 indriving operation, the capacitor 14 must be connected between portionsof the power lines Vcc and Vss which are located near the drive circuit13. Without this capacitor 14, when a plurality of capacitive loadssimultaneously operate, a voltage drop occurs due to the impedance ofthe power lines Vcc and Vss, resulting in an operation error. Inaddition, this abrupt voltage drop causes unnecessary high-frequencyelectric waves. For this reason, the coupler section 15 is set in aclosed state at a time when the head unit 11 is incorporated in theprinter and actually used.

[0039] At a time of inspecting a defect in the head unit 11, thewaveform of a current flowing through the power line Vcc must bedetected with the current probe 211 of the oscilloscope 21. If thecapacitor 14 connected between the power lines Vcc and Vss is present,most of the current is absorbed by the capacitor 14. As a result, nocurrent waveform can be detected. Therefore, the coupler section 15 isset in an open state to disconnect the capacitor 14 at the time ofdefect inspection. No problem is posed due to disconnection of thecapacitor 14, since the inspection is performed by sequentially drivingthe capacitive loads and a large amount of current is not requiredcontrary to the case where the capacitive loads are simultaneouslydriven. In addition, the capacitor 20 of the inspection apparatusperforms the same function as that of the capacitor 14 to a certaindegree in place of the capacitor 14.

[0040] As shown in FIG. 3, the control circuit 18 includes a clockgenerator 181, first, second, and third counters 182, 183, and 184,register 185, first, second, third, and fourth comparators 186, 187,188, and 189, first and second flip-flops 190 and 191, 2-input OR gates192 and 193, 2-input AND gate 194, 2-input NAND gate 195, and a selectorcircuit of a resistor 196 and switch 197 connected in series between a5-V and ground terminals Vdd and Vss of the power source 19. A clockfrom the clock generator 181 is supplied as a system clock SC to thecounters 182, 183, and 184, the flip-flops 190 and 191, and 2-input NANDgate 195.

[0041]FIGS. 4 and 5 show the operation of the control circuit 18. Thecontrol circuit 18 is initialized when the switch 187 is in an OFFstate. When the switch 197 is turned on, the control circuit 18 startsoperating. The first counter 182 determines the driving cycle of eachcapacitive load of the ink-jet print head 12. Data is shifted in thefirst half period of the operation of the first counter 182. In thesecond half period, an enable signal is set at high level to drive thecapacitive loads selected by the data items.

[0042] The fourth comparator 189 sets the second flip-flop 191 at thefirst count of the first counter 182. When the second flip-flop 191 isset, the second counter 183 starts counting. At the same time, a shiftclock is output through the NAND gate 195.

[0043] A value “n−1” is set in the register 185 in advance. At first,the second comparator 187 detects no coincidence, and the third counter184 does not operate. Consequently, the output from the third counter184 is kept at “0”. Since this value coincides with the initial value ofthe second counter 183, the data output from the first comparator 186 is“1”. The output from this counter 183 becomes “1” at the first count ofthe second counter 183. However, since the output from the third counter184 remains “0”, the first comparator 186 detects no coincidence, andthe output data becomes “0”.

[0044] Subsequently, when the second counter 183 continues countingoperation in accordance with the system clock from the clock generator181, and the counter value of the second counter 183 reaches “n−1”, thesecond comparator 187 detects a coincidence, and the third counter 184is incremented by one. At the same time, the first flip-flop 190 is setto output a latch signal. In addition, the second flip-flop 191 is resetto clear the second counter 183. In addition, the NAND gate 195 isdisabled to stop outputting the shift clock. Since the output from thethird counter 184 is “1”, the third comparator 188 detects nocoincidence, and the AND gate 194 is disabled.

[0045] When the most significant bit of the Q output from the firstcounter 182 become “1”, and the second half counting operation starts,an enable signal is output. The drive circuit 13 generates a head drivevoltage waveform for driving the nth capacitive load and supplies it tothe ink-jet print head 12.

[0046] When the first counter 182 completes one cycle of countingoperation, the circuit state is restored to the initial state. At thistime, since only the third counter 184 is kept incremented by one andnot cleared, the first comparator 186 detects a coincidence at thesecond clock of the shift clock signal in the next cycle. As aconsequence, the data “1” is output from the comparator 186. When,therefore, the first counter 182 starts the second half countingoperation in the next cycle, and an enable signal is output, the drivecircuit 13 generates a head drive voltage waveform for driving the(n−1)th capacitive load and supplies it to the ink-jet print head 12.

[0047] In this manner, the nth, (n−1)th, (n−2)th, . . . , 3rd, 2nd, and1st capacitive loads are sequentially driven in the order named. Thatis, only one capacitive load is driven at a time. Since the value of thethird counter 184 is “n−1” while the data for driving the firstcapacitive load is shifted, the third comparator 188 detects acoincidence. When the data transfer is complete in this state, and thesecond comparator 187 detects a coincidence, the AND gate 194 isenabled, and the third counter 184 is cleared.

[0048] After the first capacitive load is driven in this manner, theinitial state is restored. If, therefore, the operation of this controlcircuit 18 is continued, the respective capacitive loads of the ink-jetprint head 12 are repeatedly driven in the order of the nth, (n−1)th,(n−2)th, . . . , 3rd, 2nd, 1st, nth, (n−1)th, (n−2)th, . . .

[0049] The capacitive loads of the ink-jet print head 12 can thereforebe sequentially driven for an inspection by setting the coupler section15 in an open state and driving the control circuit 18. When theoscilloscope 21 is operated in response to the enable signal output fromthe control circuit 18 and serving as a trigger, and the waveforms of acurrent flowing in the power line Vcc are detected and observed with thecurrent probe 211 during such an inspection, the waveforms like the onesshown in FIGS. 6A to 6D are obtained.

[0050] Assume that all the capacitive loads are normal and have the samecapacitance and equivalent series resistance, the ink-jet print head 12is normally connected to the drive circuit 13, and the drive circuit 13is normal. In this case, the driving waveforms detected from the 1st tonth capacitive loads are superimposed and displayed as one waveform h1on the screen of the oscilloscope 21. That is, a waveform like the oneshown in FIG. 6A is displayed. This waveform is the waveform of a chargecurrent to each capacitive load. This waveform has a bound, which isproduced owing to resonance between the inductance component of eachwiring line and the capacitance of each capacitive load.

[0051] If, however, there is a disconnected one of the 1st to nthcapacitive loads, disconnected wiring electrode for the capacitive load,or part of the drive circuit 13 that is not operable with respect to therelated capacitive load, an observation waveform like the one shown inFIG. 6B is displayed on the screen of the oscilloscope 21. Morespecifically, if there is a portion such as a disconnected one of the1st to nth capacitive loads, disconnected wiring electrode for thecapacitive load, or part of the drive circuit 13 that is not operablewith respect to the related capacitive load, a line spectrum on theoscilloscope 21 which corresponds to the above portion becomes a groundline h2 and is displayed along with the normal waveform h1.

[0052] In contrast to this, if there is a short-circuited one of the 1stto nth capacitive loads, an observation waveform like the one shown inFIG. 6C is displayed on the screen of the oscilloscope 21. That is, aline spectrum on the oscilloscope 21 which corresponds to theshort-circuited capacitive load changes to a stepped waveform h3. Thiswaveform is displayed along with the normal waveform h1. If there areboth a disconnected capacitive load and a short-circuited capacitiveload, an observation waveform like the one shown in FIG. 6D isdisplayed.

[0053] Such observation waveforms can be easily discriminated on theoscilloscope 21. Note that observation waveforms can also be digitizedto be automatically determined.

[0054] In the head unit 11 described above, the coupler section 15 isset in an open state at the defect inspection time, so that the waveformof a current flowing in the power line Vcc can be detected and observedwith the oscilloscope 21 without being influenced by the capacitor 14.Accordingly, it is possible to accurately determine whether the headunit is nondefective or defective. After the inspection, the couplersection 15 is set in a closed state, so that the capacitor 14 can supplya current required by the drive circuit 13 for charging the capacitiveloads. Thus, the head unit 11 can perform a reliable operation withoutdifficulty.

[0055]FIG. 7 shows the detailed arrangement of the head unit 11.

[0056] In the head unit 11, the connector 16 and the drive circuit 13 ofan IC form are respectively mounted on one end and the other end of thesurface of a printed board 22 and the output terminals of the drivecircuit 13 are connected to lead or wiring electrodes of the print head12 by wiring lines 23 such as TABs, bonding wires, or the like.

[0057] The printed board 22 has a pattern of power lines Vcc and Vssconnected at one end to the connector 16 and at the other end to theinput terminals of the drive circuit 13, and the capacitor 14 isconnected in series with the coupler section 15 between the power linesVcc and Vss on the printed board 22. The coupler section 15 is formed ofa wiring pattern connected between the capacitor 14 and the power linesVcc and Vss, and a slide switch 151 inserted in the wiring pattern.

[0058] The printed board 22 further has a pattern of signal lines forthe enable signal, latch signal, data signal, and shift clock signal anda pattern of the 5-V power lines Vdd and Vss. These lines are connectedat one end to the connector 16 and at the other end to the inputterminals of the drive circuit 13.

[0059] By using the slide switch 151 in the coupler section 15, anoperation of switching between connection and disconnection of thecapacitor 14 is facilitated.

[0060]FIG. 8 shows a first modification of the head unit 11 shown inFIG. 7. In this modification, the coupler section 15 is formed of awiring pattern connected between the capacitor 14 and the power linesVcc and Vss, and a connector 152 a inserted in the wiring pattern andhaving a pair of open terminals to be short-circuited by a short-circuitplug 152 b. The arrangement in FIG. 8 is the same as that shown in FIG.7 except for this.

[0061] In this case, the short-circuit plug 152 b is removed in aninspection, and is set to be in contact with the open terminals of theconnector 152 a to connect the capacitor 14 after the inspection.

[0062] With this arrangement, the cost of the coupler section 15 can befurther reduced. In addition, whether an inspection is complete can beeasily determined by checking the presence and absence of theshort-circuit plug 152 b.

[0063]FIG. 9 shows a second modification of the head unit 11 shown inFIG. 7. In this modification, the coupler section 15 is formed of awiring pattern which is connected between the capacitor 14 and the powerlines Vcc and Vss and has a pair of open terminals 153 to beshort-circuited by solder. The arrangement shown in FIG. 9 is the sameas that shown in FIG. 7 except for this.

[0064] In this case, an inspection is performed by using the head unit11 without any change. After the inspection, the open terminals 153 areshort-circuited by solder applied in a bridge connection form.

[0065] With this arrangement, the cost of the coupler section 15 can befurther reduced, and the area occupied by the section 15 can be reduced.

[0066]FIG. 10 shows a third modification of the head unit 11 shown inFIG. 7. In this modification, the coupler section 15 is formed of awiring pattern which is connected between the capacitor 14 and the powerlines Vcc and Vss and has a pair of open terminals 154 a to beshort-circuited by a chip jumper element 154 b having a resistance of0Ω. The arrangement shown in FIG. 10 is the same as that shown in FIG. 7except for this.

[0067] In this case, the chip jumper element 154 b is removed in aninspection. After the inspection, the chip jumper element 154 b ismounted on the open terminals 154 a by soldering to short-circuit theopen terminals 154 a, thereby connecting the capacitor 14.

[0068] In this arrangement, when the process yield improves so that noinspection is required, the chip jumper element 154 b can beautomatically mounted, together with other circuit components in theassembling step. In addition, when the yield decreases for some reason,the chip jumper element 154 b may not be mounted in the assembling stepand may be mounted after the inspection. As described above, thisarrangement exhibits flexibility in execution and non-execution of aninspection.

[0069]FIG. 11 shows a fourth modification of the head unit 11 shown inFIG. 7. In this modification, the coupler section 15 is formed of awiring pattern which is connected between one end of capacitor 14 and apredetermined connection terminal of the connector 16, for example, nextto a connection terminal connected to the power line Vcc and located onthe left end of the connector 16 and between the other end of thecapacitor 14 and the power line Vss. In an inspection, the connector 16of the head unit 11 is connected to a control circuit of the inspectionapparatus by an inspection cable 31 shown in FIG. 12A or inspectioncable 32 shown in FIG. 12B. After the inspection, the connector 16 ofthe head unit 11 is connected to a control circuit incorporated in aproduced ink-jet printer by a product cable 33 shown in FIG. 12C.

[0070] The inspection cable 31 has a wiring line L2 connected to theconnection terminal of the connector 16 for the power line Vcc, and awiring line L1 connected to the connection terminal of the connector 16which is located next to the connection terminal for the power line Vcc.Since the wiring line L1 is interrupted and spaced from the wiring lineL2, the capacitor 14 is not electrically connected to the power line Vccwhen the inspection cable 31 is used. In driving operation, a currentflowing through the wiring line L2 can be measured to detect a waveformof the current without any influence of the capacitor 14.

[0071] On the other hand, the inspection cable 32 also has a line L3connected to the connection terminal on the left end of the connector 16for the power line Vcc and a wiring line L4 connected to a connectionterminal of the connector 16 which is next to the connection terminalfor the power line Vcc. A current measuring point on the wiring line L3is located closer to the connector 16 and more distant from the powersource than a point where the wiring lines L4 is connected to the wiringline L3. Therefore, when the inspection cable 32 is used, the capacitor14 is electrically connected to the power line Vcc. However, in drivingoperation, a current flowing through the wiring line L3 can be alsomeasured to detect a waveform of the current without any influence ofthe capacitor 14.

[0072] Moreover, the product cable 33 has also has a line L5 connectedto the connection terminal on the left end of the connector 16 for thepower line Vcc and a wiring line L6 connected to a connection terminalof the connector 16 which is next to the connection terminal for thepower line Vcc. Further, the wiring lines L5 and L6 are connected toeach other near the connector 16. When, therefore, the inspection iscomplete, and the product cable 33 is used to connect the controlcircuit to the connector 16 in the assembling step for obtaining aproduct, the capacitor 14 is electrically connected to the power lineVcc at a short wiring length. This allows the capacitor 14 to supply acurrent required for instantaneously charging the capacitive loads tothe drive circuit 13 in driving operation.

[0073] By changing the types of cables, i.e., the inspection and productcables, to be connected to the connector 16 of the head unit 11 in thismanner, the inactive and active states of the function of the capacitor14 can be automatically switched.

[0074]FIG. 13 shows a fifth modification of the head unit 11 shown inFIG. 7. FIG. 14 shows a state where a capacitor is mounted on the headunit shown in FIG. 13. As shown in FIG. 13, the head unit 11 has pads 34for receiving the capacitor 14 to be mounted. A defect inspection isperformed in the state shown in FIG. 13. After the inspection iscomplete, as shown in FIG. 14, the capacitor 14 is mounted on the pads34 and connected to the power lines Vcc and Vss.

[0075] With this arrangement, since the capacitor 14 must be mountedafter the inspection, the number of assembly steps increases. However, acurrent waveform can be easily detected without forming any specialcomponent. This arrangement is therefore effective when the quantity ofproducts is small.

[0076] In addition, according to the inspection method of thisembodiment, since a power source current supplied to the drive circuitis detected, the drive circuit ground need not be separated from thehead ground. This method can therefore be applied to drive circuits forvarious types of ink-jet print heads.

[0077]FIG. 15 shows an equivalent circuit diagram of the ink-jet printhead 12. In the print head 12, one ends of the capacitive loads 12 a areconnected commonly to the power line Vcc, and the other ends of thecapacitive loads 12 a are connected to output terminals 131 a of a drivecircuit 131.

[0078]FIG. 16 shows an equivalent circuit diagram of the firstmodification of the ink-jet print head 12 shown in FIG. 1. In thismodification, the capacitive loads 12 a are connected in series and eachof the capacitive loads 12 a is connected between adjacent two of theoutput terminals 132 a of a drive circuit 132.

[0079]FIG. 17 shows an equivalent circuit diagram of the secondmodification of the ink-jet print head 12 shown in FIG. 1. In thismodification, the capacitive loads 12 a are arranged to form a matrixcircuit 24. One ends of the capacitive loads 12 a are connected tooutput terminals 1331 a of an output port 1331 of a drive circuit 133,and the other ends of the capacitive loads 12 a are connected to outputterminals 1332 a of an output port 1332 of the drive circuit 133.

[0080] The coupler section 15 is opened and the capacitor 14 isdisconnected from each of the drive circuits 131, 132, and 133 for therespective types of ink-jet print heads shown in FIGS. 15, 16, and 17.The respective capacitive loads 12 a are then sequentially driven, andthe waveform of a current flowing through the power line Vcc isdetected. By observing the current waveform with the oscilloscope 21,the user can determine whether the head unit is nondefective ordefective.

[0081] In this embodiment, measurement is performed by detecting thewaveform of a current flowing through the power line Vcc. However, thesame result can be obtained when measurement is performed by detectingthe waveform of a current flowing through the power line Vss.

[0082] In addition, in this embodiment, the waveform of a currentflowing through the power line Vcc is detected with the current probe211. However, the present invention is not limited to this. FIG. 18shows a first modification of the inspection apparatus shown in FIG. 1.As shown in FIG. 18, when a resistor 25 having a resistance low enoughto exert no influence on the operation is inserted in series in thepower line Vss, and a voltage drop across the resistor 25 is measured,the same result can be obtained because a current waveform can beindirectly detected.

[0083] In this embodiment, an inspection is performed by directly orindirectly detecting a current waveform. However, the present inventionis not limited to this, and an inspection may be performed by directlydetecting a voltage variation waveform. FIG. 19 shows a secondmodification of the inspection apparatus shown in FIG. 1. If, forexample, the output voltage form the voltage regulated power source 17is stable, a power source voltage between the power lines Vcc and Vssmay be directly detected to measure a power source voltage variationwaveform. In this case, the resistor 25 is connected in series in thepower line Vcc. That is, the inspection is not limited to detecting acurrent waveform as a waveform to be detected as long as nondefectiveand defective products can be discriminated. In addition, an inductor,semiconductor element, or the like, or an arbitrary impedance circuitformed by using a combination thereof may be used in place of theresistor 25 to improve the detection sensitivity or change a detectionwaveform into a waveform that can be easily discriminated.

[0084] In this embodiment, the capacitive loads 12 a of the print head12 are sequentially driven, from the nth load to the 1st load. When allthe capacitive loads are driven, this operation is repeated. With thiscontrol, as the oscilloscope 21, an analog non-storage oscilloscope canbe used. Since the repetition display cycle of the analog oscilloscopeis shorter than that of the digital oscilloscope, the speed of clocksfrom a clock generator 181 can be increased. This can shorten the timerequired for an inspection.

[0085] In inspecting the head unit 11, only a small number of capacitiveloads are detected as defective elements when they are driven. When,therefore, current waveforms are observed with an analog oscilloscope,normal waveforms are repeatedly displayed in most cases, whereas thenumber of times abnormal waveforms are displayed is very small. For thisreason, the luminance of displayed normal waveforms is high, but theluminance of displayed abnormal waveforms is very low. Under thecircumstances, the observer of the oscilloscope may miss an abnormalwaveform. To solve this problem, an analog oscilloscope of anafter-image type may be used.

[0086] As the oscilloscope 21, a digital storage oscilloscope can alsobe used. In general, the digital storage oscilloscope displays waveformswith a constant luminance regardless of the frequencies of occurrence.Therefore, the luminance of an abnormal waveform with a low frequencyoccurrence does not decrease. This allows reliable observation ofabnormal waveforms. In the use of the digital storage oscilloscope, eachof n capacitive loads may be driven once.

[0087] To execute this operation, the control circuit 18 in FIG. 3 maybe changed such that when each of the n capacitive loads is completelydriven once, and an AND gate 194 is enabled, the operation of thecircuit is stopped instead of being repeated. When the control circuit18 completely drives each of the n capacitive loads once and stopsoperating, a display means such as an LED may be used to notify theoperator of the end of the operation. With this operation, the operatorcan easily know the end of the inspection.

[0088] Since the waveform repetition display cycle of the digitalstorage oscilloscope is long, the time intervals at which the respectivecapacitive loads are sequentially driven must be set to be long inaccordance with this cycle. In addition, since the digital storageoscilloscope can digitally process received current waveforms, whether areceived current waveform falls within a normal waveform range can beautomatically determined by digital processing instead of beingdetermined by the operator upon observation of the displayed contents.In this case, since current waveforms need not be displayed on thescreen, the oscilloscope need not be used as a current waveformrecording means, and a digital memory having the function of recordingcurrent waveforms may be used.

[0089] In this embodiment, the waveforms observed when the respectivecapacitive loads are driven are superimposed and displayed on the screenof the oscilloscope 21, and the operator checks the waveform deviationbetween a normal waveform and abnormal waveform to determine whether thehead unit is nondefective or defective. However, the present inventionis not limited to this. A normal waveform range may be marked on thescreen of the oscilloscope 21 in advance, and whether a head unit isnondefective or defect may be determined by checking whether eachwaveform falls within the marked range. Marking may be performed byusing the cursor display function of the oscilloscope. A simpler way isto directly mark a range on the screen with a marker pen or the like.

[0090] A case wherein a normal waveform range is marked on the screen ofthe oscilloscope 21 will be described below. For example, this range ismarked as indicated by oblique lines a in FIGS. 20A to 20G. With thismarking, when all the capacitive loads are normal and have the samecapacitance and equivalent series resistance, and the drive circuit 13and the connection between the print head 12 and drive circuit 13 arenormal, the waveforms detected when the 1st to nth capacitive loads aredriven superimposed and displayed as one waveform h1 on the screen ofthe oscilloscope 21 within the marked range, as shown in FIG. 20A.

[0091] If, however, there is a disconnected one of the 1st to nthcapacitive loads, disconnected wiring electrode for the capacitive load,or part of the drive circuit 13 that is not operable with respect to therelated capacitive load, the emission lines of the normal drivingwaveform h1 and waveform h2 of a ground line are superimposed anddisplayed on the screen of the oscilloscope 21, as shown in FIG. 20B.The waveform h2 of the ground line falls outside the marked range at theposition of a peak of the normal driving waveform h1.

[0092] In contrast to this, if there is a short-circuited one of the 1stto nth capacitive loads, the emission lines of the normal drivingwaveform h1 and waveform h3 that rises in a stepped form aresuperimposed and displayed on the screen of the oscilloscope 21, asshown in FIG. 20C. In this case, the waveform h3 falls outside themarked range.

[0093] If there are a disconnected capacitive load and a short-circuitedcapacitive load among the 1st to nth capacitive loads, the emissionlines of the normal driving waveform h1, waveform h2 of the ground line,and waveform h3 that rises in a stepped form are superimposed anddisplayed on the screen of the oscilloscope 21, as shown in FIG. 20D. Inthis case, both the waveforms h2 and h3 fall outside the marked range.

[0094] If there is a capacitive load having an excessively smallcapacitance among the 1st to nth capacitive loads, or a connectionelectrode or drive circuit for a capacitive load which has anexcessively large circuit resistance, the normal driving waveform h1 andan abnormal waveform h4 with a small current are superimposed anddisplayed on the screen of the oscilloscope 21, as shown in FIG. 20E. Inthis case, the abnormal waveform h4 falls outside the marked range.

[0095] If there is a capacitive load having an excessively largecapacitance among the 1st to nth capacitive loads, the normal drivingwaveform h1 and an abnormal waveform h5 with a large current aresuperimposed and displayed on the screen of the oscilloscope 21, asshown in FIG. 20F. In this case, the abnormal waveform h5 falls outsidethe marked range.

[0096] If there is a drive circuit having an excessively large delay,the normal driving waveform h1 and an abnormal waveform h6 with a delayare superimposed and displayed on the screen of the oscilloscope 21, asshown in FIG. 20G. In this case, the abnormal waveform h6 falls outsidethe marked range.

[0097] As described above, nondefective and defective determination oneach head unit can be accurately performed by using the markings on thescreen of the oscilloscope 21.

[0098] In addition, if the waveforms detected when the respectivecapacitive loads are driven are superimposed and displayed on the screenof the oscilloscope 21 with the normal range being marked on the screenof the oscilloscope 21, the operator can easily find an abnormal drivingwaveform. This allows the operator to more accurately determine whethera head unit is nondefective or defective. Note that when the waveformsdetected when the respective capacitive loads are driven aresuperimposed and displayed on the screen of the oscilloscope 21, adynamic characteristic defect including information about a change overtime, like the one shown in FIG. 20G, can be accurately detected.

[0099] In the control circuit 18 in FIG. 3, a preset register may beused instead of clearing the third counter 184. A value “n−m” is presetin the third counter 184 by using this present register. In this case,when the comparison input terminals of the third comparator 188 whichare connected to the register 185 are connected to another register, anda value “n−k” is set in the register in advance, only kth to mth(n≧m≧k≧1) capacitive loads can be sequentially driven. With thiscontrol, if a defective capacitive load is detected in the firstinspection, an analysis can be made to specify the position of thedefective capacitive load.

[0100] The above embodiment has exemplified the case wherein the ink-jetprint head using electrostrictive members as capacitive loads is driven.However, the present invention is not limited to this and can be appliedto a case wherein an apparatus using liquid crystal elements, which arecapacitive loads like electrostrictive members, EL print head, and thelike are driven as capacitive loads.

[0101] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

1. An inspection method for a capacitive load driving unit whichcomprises a plurality of capacitive loads, a drive circuit for chargingand discharging said capacitive loads, a pair of power lines forsupplying a drive voltage from an external power source to said drivecircuit, and a current reserve circuit for reserving a current requiredby said drive circuit for charging said capacitive loads, wherein saidcurrent reserve circuit includes a capacitor that is charged by thedrive voltage from said power lines, and a coupler section for couplingthe capacitor between the power lines except for a defect inspection,said method comprising the steps of: controlling said drive circuit tosequentially drive said plurality of capacitive loads in a state wheresaid capacitor is disconnected from said power lines; and detecting anelectric change that occurs on said power line upon driving each of saidcapacitive loads to determine whether or not a defect is present.
 2. Aninspection method for a capacitive load driving unit which comprises aplurality of capacitive loads, a drive circuit for charging anddischarging said capacitive loads, and a pair of power lines forsupplying a drive voltage from an external power source to said drivecircuit, said method comprising the steps of: controlling said drivecircuit to sequentially drive said plurality of capacitive loads; anddetecting an electric change that occurs on said power line upon drivingeach of said capacitive loads to determine whether or not a defect ispresent; wherein said detecting step includes a step of superimposingand recording waveforms the electric change in synchronism with adriving timing of each of said capacitive loads to recognize, as thepresence of a defect, a state in which the recorded waveform fallsoutside a predetermined allowable range.
 3. An inspection method for acapacitive load driving unit which comprises a plurality of capacitiveloads, a drive circuit for charging and discharging said capacitiveloads, and a pair of power lines for supplying a drive voltage from anexternal power source to said drive circuit, said method comprising thesteps of: controlling said drive circuit to sequentially drive saidplurality of capacitive loads; and detecting an electric change thatoccurs on said power line upon driving each of said capacitive loads todetermine whether or not a defect is present; wherein said detectingstep includes a step of superimposing and recording waveforms theelectric change as displayed after-images on an oscilloscope insynchronism with a driving timing of each of said capacitive loads torecognize, as the presence of a defect, a state in which the recordedwaveform falls outside a predetermined allowable range.
 4. An inspectionapparatus for a capacitive load driving unit which comprises a pluralityof capacitive loads, a drive circuit for charging and discharging saidcapacitive loads, a pair of power lines for supplying a drive voltagefrom an external power source to said drive circuit, and a currentreserve circuit for reserving a current required by said drive circuitfor charging said capacitive loads, wherein said current reserve circuitincludes a capacitor that is charged by the drive voltage from saidpower lines, and a coupler section for coupling the capacitor betweenthe power lines except for a defect inspection, said apparatuscomprising: a control circuit for controlling said drive circuit tosequentially drive said plurality of capacitive loads in a state wheresaid capacitor is disconnected from said power lines; and a detectingcircuit for detecting an electric change that occurs on said power lineupon driving each of said capacitive loads to determine whether or not adefect is present.
 5. An inspection apparatus for a capacitive loaddriving unit which comprises a plurality of capacitive loads, a drivecircuit for charging and discharging said capacitive loads, and a pairof power lines for supplying a drive voltage from an external powersource to said drive circuit, said apparatus comprising: a controlcircuit for controlling said drive circuit to sequentially drive saidplurality of capacitive loads; and a detecting circuit for detecting anelectric change that occurs on said power line upon driving each of saidcapacitive loads to determine whether or not a defect is present;wherein said detecting circuit includes a recording section forsuperimposing and recording waveforms the electric change in synchronismwith a driving timing of each of said capacitive loads to recognize, asthe presence of a defect, a state in which the recorded waveform fallsoutside a predetermined allowable range.
 6. An inspection apparatus fora capacitive load driving unit which comprises a plurality of capacitiveloads, a drive circuit for charging and discharging said capacitiveloads, and a pair of power lines for supplying a drive voltage from anexternal power source to said drive circuit, said apparatus comprising:a control circuit for controlling said drive circuit to sequentiallydrive said plurality of capacitive loads; and a detecting circuit fordetecting an electric change that occurs on said power line upon drivingeach of said capacitive loads to determine whether or not a defect ispresent; wherein said control circuit includes a signal generatingsection for sequentially generating trigger signals which respectivelyenable said capacitive loads to be driven, and said detecting circuitincludes an oscilloscope for superimposing and recording waveforms ofthe electric change in response to each of the trigger signals torecognize, as the presence of a defect, a state in which the recordedwaveform falls outside a predetermined allowable range.
 7. An inspectionapparatus according to claim 4, wherein said oscilloscope is of adigital storage type, and said control circuit includes notifying meansfor notifying the end of the defect inspection after the trigger signalshave been generated for all the capacitive loads.
 8. An inspectionapparatus according to claim 6, wherein said oscilloscope is of adigital storage type, and said detecting circuit includes adetermination circuit for determining a state in which the a waveform ofthe electric change falls outside a predetermined allowable range, asthe presence of a defect.